U.S. Pat. No. 3,755,477 describes a process for producing fluorinated aliphatic hydrocarbons which comprises fluorinating a halogenated aliphatic hydrocarbon, including tetrachloroethylene and chlorotrifluoroethylene, by reaction in the gas phase with HF in the presence of a steam-treated and calcined chromium oxide catalyst prepared by a multi-step process. Example 23, column 5, shows tetrachloroethylene as a raw material with formation of CF.sub.3 CHCl.sub.2 (20%), CF.sub.3 CHClF (20%), CF.sub.3 CHF.sub.2 (30%), and CF.sub.3 CClF.sub.2 (20%) at 10/1 HF/C.sub.2 Cl.sub.4 mole ratio, 5.4 seconds contact time and 350.degree. C. reaction temperature. Example 25 shows that, with CF.sub.3 CHCl.sub.2 as starting material at 390.degree. C., CF.sub.3 CHClF (21%) and CF.sub.3 CHF.sub.2 (67%) are produced, but again these desirable hydrogen-containing products are accompanied by perhalogenated CF.sub.3 CClF.sub.2 (2.5%). The formation of CF.sub.3 CClF.sub.2, 20% in 24 and 2.5% in 25, is objectionable; not only does it constitute a yield loss of the hydrogen-containing substances, but CF.sub.3 CClF.sub.2 is extremely close boiling to CF.sub.3 CHF.sub.2 with the resulting mix being costly to separate.
U.S. Pat. No. 3,258,500 describes a process for the catalytic vapor-phase reaction of HF with halohydrocarbons employing a catalyst that consists essentially of a heat-activated anhydrous chromium (III) oxide which may be supported on alumina. Example 17, column 14, shows that fluorination of tetrachloroethylene with this catalyst at 400.degree. C. produces 35.0% pentafluoroethane, 9.2% 1,2-tetrafluorochloroethane, and 3.5% 2,2-dichloro-1,1,1-trifluoroethane. At 300.degree. C. the product distribution is 38.3% 1,2-tetrafluorochloroethane, 25.4% pentafluoroethane, and 16.0% 2,2-dichloro-1,1,1-trifluoroethane. Example 20, column 19, shows that chlorotrifluoroethylene at 400.degree. C. yields 27% CF.sub.3 CHF.sub.2. It can be seen that, although the yield of the hydrogen-containing products is high, that of pentafluoroethane does not exceed about 35% even at temperatures as high as 400.degree. C.
U.S. Pat. No. 4,843,181 discloses a gas phase process for the manufacture of CF.sub.3 CHCl.sub.2 and/or CF.sub.3 CHClF by contacting a suitable tetrahaloethylene and/or pentahaloethane with HF in the presence of Cr.sub.2 O.sub.3 prepared by pyrolysis of (NH.sub.4).sub.2 Cr.sub.2 O.sub.7, the reaction being conducted under controlled conditions whereby the production of CF.sub.3 CHF.sub.2 is minimized.
Canadian Patent No. 849,024 also discloses the formation of CF.sub.3 CHF.sub.2 by reaction of HF with a perhaloethylene, e.g., C.sub.2 Cl.sub.4, CCl.sub.2 .dbd.CF.sub.2 and CClF.dbd.CF.sub.2, over a hydrous Cr.sub.2 O.sub.3 as catalyst, but the CF.sub.3 CHF.sub.2 yields are relatively poor.
Canadian Patent No. 1,196,345 discloses that addition of HF to perfluoroethylene over a chromium oxyfluoride as catalyst, which has been activated by treatment with a mixture of HF and F.sub.2, produces CF.sub.3 CHF.sub.2 in high yields. Much inferior yields are obtained using a comparable catalyst activated with HF alone. The disclosed process suffers in that the catalyst activation step not only involves the use of expensive and hazardous molecular fluorine, but requires a back-up scrubbing solution of hexafluoropropylene trimer to scavenge unreacted F.sub.2. All this represents a loss of F.sub.2 and catalyst activation potential, and further adds to the cost of the process.
The prior art, in general, has shown that CF.sub.3 CHCl.sub.2 is obtainable in good yields from tetrachloroethylene, a readily available and relatively inexpensive commodity. However, CF.sub.3 CHCl.sub.2 as starting material for the production of highly fluorinated hydrogen-containing products by reaction with HF also yields perhalo by-products, evidently via chlorination side reactions, especially when an attempt is made to increase conversion to the desired hydrogen-containing products by operating at high temperatures.